PUBLIC INTERFACE

Return the boundary field. The boundary is returned exactly
as given in the "Content-type:" field; that is, the leading
double-hyphen ("--") is not prepended.

(Well, almost exactly... from RFC 2046:

(If a boundary appears to end with white space, the white space
must be presumed to have been added by a gateway, and must be deleted.)

so we oblige and remove any trailing spaces.)

Returns the empty string if there is no boundary, or if the boundary is
illegal (e.g., if it is empty after all trailing whitespace has been
removed).

multipart_boundary

Like "boundary()", except that this will also return the empty
string if the message is not a multipart message. In other words,
there's an automatic sanity check.

type

Try real hard to determine the content type (e.g., "text/plain",
"image/gif", "x-weird-type", which is returned
in all-lowercase.

A happy thing: the following code will work just as you would want,
even if there's no subtype (as in "x-weird-type")... in such a case,
the $subtype would simply be the empty string:

($type, $subtype) = split('/', $head->mime_type);

If the content-type information is missing, it defaults to "text/plain",
as per RFC 2045:

Default RFC 2822 messages are typed by this protocol as plain text in
the US-ASCII character set, which can be explicitly specified as
"Content-type: text/plain; charset=us-ascii". If no Content-Type is
specified, this default is assumed.

Note: under the ``be liberal in what we accept'' principle, this routine
no longer syntax-checks the content type. If it ain't empty,
just downcase and return it.

NOTES

Since nearly all (if not all) parameters must have non-empty values
to be considered valid, we just return the empty string to signify
missing fields. If you need to get the real underlying value,
use the inherited "param()" method (which returns undef if the
parameter is missing).